SPRING WIRE, TENSION CLAMP FORMED THEREFROM AND METHOD FOR MANUFACTURING SUCH A SPRING WIRE The invention relates to a spring wire which is manufactured from a spring steel with a carbon content of 0.35 — 0.42% by weight.
Furthermore, the invention relates to a tension clamp for holding down a rail for rail vehicles in a rail fastening point, which is formed from such a spring wire, and a method for manufacturing a spring wire of the type in guestion here.
In a “rail fastening point”, the rail to be fastened in each case is fastened to the substrate, which carries the track to which the rail belongs.
The substrate can be formed by a conventional sleeper made of wood or by sleepers or panels formed from a concrete or a plastic material.
The rail fastening point typically comprises at least one guide plate which lies laterally on the rail and dissipates the transverse forces acting on therail into the substrate during use, and a tension clamp which is tensioned against the substrate of the tension clamps.
With the end of at least one springarm, the tension clamp exerts an elastically resilient hold-down force on the rail foot, through which the rail is held against the substrate.
The hold-down forces can be applied particularly effectively by means of tension clamps shaped into a W or w shape, which act on the rail foot with the free ends of their two spring arms.
Examples of tension clamps shaped like this are the products explained under URL https://www.vossloh.com/de/produkte-und-loesungen/produktfinder/ (retrieval date 12 August 2019). The spring wires required for producing tension clamps typically have circular diameters of 9 - 15 mm.
In practical use, the individual sections of a tension clamp are either predominantly subjected to bending or torsional loads, wherein more or less strong proportions of the respective other load form can be added to the respective dominant load.
The usual production route for their manufacture comprises the work steps “casting a steel melt into bars”, “heating through the bars” and “hot rolling the bars into a spring wire”, “cooling the hot-rolled spring wire” and “laying or winding the spring wire into a coil”, wherein the hot rolling is usually carried out in several steps, which comprise pre-rolling, intermediate rolling and finish rolling the slab to form the spring wire.
The work steps to be carried out and influencing variables to be observed are known to the person skilled in the art (see for example, Stahl Fibel, 2015, Verlag Stahleisen GmbH, Diisseldorf, ISBN 978-3-514-00815-1).
The tension clamps are cold-formed from the spring wires produced in this way.
For this purpose, rods are cut to length from the spring wires, which are then usually bent in several steps to form the tension clamp.
In this way, it is possible to produce tension clamps of complex shapes.
The tension clamps obtained are finally subjected to a heat treatment in which they are heated to a temperature above Ac3 and then guenched in order to optimize their mechanical properties by hardening.
The aim is to set high tensile strengths Rm and high yield strengths Rp0.2. In this case, a ratio Rm/Rp0.2 of =
1issoughtin order, on the one hand, to be able to apply high elastic hold-down forces with the tension clamps and, on the other hand, to extend the region of the elastic deformability of the tension clamp and thus its fatigue strength as far as possible.
Typically, the tensile strengths Rm and yield strengths Rp0.2 for tension clamps of the type in question here are in the range of 1200 — 1400 MPa.
An increase in strength through, for example, an increase in the carbon content is limited here by the reguirement that the spring wire should still be cold-formed.
A steel proven in practice for the manufacture of spring wires for tension clamps, standardized in accordance with DIN EN 10089:2002 under the designation "38Si7”
and recorded in the steel iron list (“Stahl-Eisen-Liste”) with the material number
1.5023, therefore consists of, in % by weight, 0.35 - 0.42% C, 1.50 — 1.80% Si, 0.50 -
0.80% Mn and as the remainder of iron and unavoidable impurities, wherein unavoidable impurities include up to 0.025% P and up to 0.025% S. In addition to the alloying measures, the mechanical properties of a spring wire provided for the manufacture of spring elements can also be improved by so-called “thermomechanical rolling”. In the case of a variant of such thermomechanical rolling aimed in particular at spring wire, which is provided for the manufacture of flex-loaded springs, the spring wire is hot-rolled in a temperature range in which its microstructure is not yet completely recrystallised, but which is above the Ar3 temperature of the steel. In this way, spring wires with particularly fine microstructures can be produced, which contributes to a high strength and optimized spring behaviour of the tension clamp (DE 195 46 204 C1). In the case of another variant of thermomechanical forming aimed in particular at the treatment of spring — wire, which is provided for the manufacture of torsionally-loaded springs, the rod- shaped starting material is heated at a heating rate of at least 50 K/s to a temperature above the recrystallisation temperature and then formed at a temperature at which dynamic and/or static recrystallisation of the austenite results. The austenite of the formed product recrystallised in this way is quenched and tempered (DE 198 39 383
Al). In addition to the prior art explained above, the spring steel described in CN 105 112 774 Ais also to be mentioned, which can be hardened by air cooling and should have a high deformability at a comparatively low content of carbon and microalloying elements. For this purpose, this known spring steel consists of, in % by weight, 0.15 -
0.50% C, 0.30 - 2.00% Si, 0.60 — 2.50% Mn, up to 0.020% S, up to 0.025% P, 0.0005 -
0.0035% B and as the remainder of Fe. After being heated to 900 - 1050*C and held at this temperature, the steel composed in this way undergoes controlled cooling to obtain a microstructure, the main components of which are bainite and martensite, and which may also have smaller amounts of residual austenite. The properties of the steel can be further improved by low temperature tempering.
The steel treated in this way should have a tensile strength Rm of at least 1350 MPa, a yield strength Rp0.2 of at least 1050 MPa and an elongation A of at least 10%. Based on the prior art explained above, the object has been to provide a spring wire which can also be cold-formed well with diameters of at least 9 mm, but which has improved mechanical properties.
According to the invention, a spring wire achieving this object has at least the features indicated in claim 1. In addition, a tension clamp with optimized properties and a method which enables — the practice-oriented production of spring wires according to the invention should be indicated.
A tension clamp for holding down rails for rail vehicles in a rail fastening point, which achieves this object, is formed from a spring wire provided according to the invention.
According to the invention, a method which achieves the above object comprises at least the work steps and features indicated in claim 14. It goes without saying that when carrying out the method according to the invention, the person skilled in the art not only carries out the method steps mentioned in the claims and explained here in detail, but also carries out all other steps and activities that are usually carried out in the practical implementation of such methods in the prior art if the necessity arises.
Advantageous embodiments of the invention are defined in the dependent claims and, like the general concept of the invention, are explained in detail in the following.
Unless explicitly stated otherwise, information on the contents of alloy constituents is always provided in % by weight in this text.
Accordingly, a spring wire according to the invention is manufactured from a steel,
which consists of, in % by weight, C: 0.35 - 0.42%, Si: 1.5 - 1.8%, Mn: 0.5 - 0.8%, 5 Cr: 0.05 - 0.25%, Nb: 0.020 - 0.10%, V: 0.020 - 0.10%, N: 0.0040 - 0.0120%, Al: < 0.03%, and as the remainder of iron and unavoidable impurities, the total content of impurities being limited to at most 0.2% and including impurities up to 0.025% P and
0.025% S. The alloy concept provided according to the invention for the spring wire is based on the fact that the tensile strength Rm and the yield strength Rp0.2 are increased by adding additional alloy elements. This allows the carbon content and the associated cold formability of the spring wire to be kept at an optimally low level for practical processing, while at the same time significantly increasing the strength Rm and yield strength Rp0.2 compared to the prior art. Specifically, the individual alloy components and their contents in the alloy of a spring wire according to the invention have been determined as follows: Carbon ("C") is present in the spring steel of a spring wire according to the invention in contents of 0.35 — 0.42% by weight in order to ensure good deformability, high toughness, good corrosion resistance and low sensitivity to stress- or hydrogen- induced cracking. C contents of at most 0.40% by weight, in particular less than 0.40% — by weight, have proven particularly effective in terms of optimized ductility and the associated optimized deformability at room temperature. Silicon ("Si") is present in the steel of a spring wire according to the invention in contents of 1.5 - 1.8% by weight, in particular 1.50 - 1.80% by weight, in order to ensure a high strength through mixed crystal solidification. In addition, the high Si content ensures good resistance ("relaxation resistance") against a decrease in the strength values of the spring wire in the course of the heat treatment, which tension clamps formed from the spring wire according to the invention regularly undergo after their cold forming. To this end, Si contents of at least 1.5% by weight are required. However, excessively high Si contents would reduce toughness, increase the risk of decarburisation during heat treatment and also contribute to coarse grain formation. Therefore, the Si content remains limited to 1.8% by weight according to the invention. Manganese ("Mn") is present in the steel of a spring wire according to the invention in contents of 0.5 — 0.8% by weight in order to ensure sufficient hardenability of the spring steel. In addition, Mn binds the sulfur, which is generally unavoidable in the steel due to the manufacturing process, to form MnS and thus prevents its harmful effect. For this purpose, atleast 0.5% by weight, in particular at least 0.50% by weight, Mn are required in the steel, wherein an optimized effect is achieved at contents of at least 0.6% by weight, in particular at least 0.60% by weight or at least
0.7% by weight. However, excessively high Mn contents would worsen the ductile- brittle transition temperature ("DBTT"), therefore the Mn content is limited to at most 0.8% by weight, in particular 0.80% by weight. Chromium ("Cr") is present in the spring steel of a spring wire according to the invention in contents of 0.05 — 0.25% in order to further improve the hardenability of the steel. In this case, the presence of Cr in the steel according to the invention ensures that the microstructure of a tension clamp formed from a spring wire according to the invention consists of more than 95% by area of martensite after hardening. A C content of atleast 0.05% by weight can also reduce the carbon activity and the risk of surface layer decarburisation during heat treatment. The positive effects of Cr in the spring steel of a spring wire according to the invention can be particularly reliably utilised in that a Cr content of at least 0.1% by weight, in particular at least 0.10% by weight or in particular at least 0.18% by weight is provided. At Cr contents above 0.25% by weight, on the other hand, there is a risk that the toughness and relaxation resistance of the spring steel would be impaired. Aluminium ("Al") is not required in the steel according to the invention for deoxidation during steel production, but can optionally be added to the spring steel in contents of up to 0.03% by weight in order to support the development of a fine-grained microstructure. Higher Al contents, however, would impair the purity of the steel of a steel according to the invention and thus its toughness through the excessive formation of Al oxides or nitrides. Niobium ("Nb") is of particular importance for the invention and in the spring steel of a spring wire according to the invention in contents of 0.02 — 0.1% by weight. Nb delays recrystallisation during a thermomechanical rolling carried out in the temperature range recrystallisation stop temperature - Ar3 temperature of the spring steel, through which a particularly fine-grained microstructure of the spring wire according to the invention is obtained. At the same time, the presence of Nb limits the grain growth if the spring wire according to the invention is heated to the austenitisation temperature during the heat treatment of the tension clamp formed from it and held there. As a result, a significant improvement in strength is achieved by the addition of Nb according to the invention and the resulting development of a particularly fine-grained microstructure, which is also maintained during the heat treatment, which a tension clamp finally passes through. In order to be able to use the positive effect of Nb particularly safely, the Nb content of the spring steel of a spring wire according to the invention can be at least 0.0250% by weight, at least 0.0280% by weight or at least
0.030% by weight. Nb can be used particularly effectively at contents of up to 0.070% by — weight, in particular up to 0.050% by weight. Vanadium ("V") is present in the spring steel of a spring wire according to the invention in contents of 0.020 - 0.10% by weight. V forms carbides and nitrides with carbon and nitrogen, which are typically present as fine, for example 8 - 12 nm, in particular about 10 nm, large carbonitride precipitates and contribute substantially to the increase of the strength of a spring wire according to the invention by precipitation hardening.
At the same time, V contributes in this manner to the relaxation resistance of the spring steel, of which a spring wire according to the invention consists.
In order to be able to use the positive effect of V particularly safely, the V content of the spring steel of a spring wire according to the invention can be at least 0.0250% by weight, at least 0.0280% by weight or at least 0.030% by weight.
V can be used particularly effectively at contents of up to 0.070% by weight, in particular up to 0.060% by weight.
The presence of Nb and V combined according to the invention results in high tensile strengths Rm and regularly approximately the same high yield strengths Rp0.2, so thatin a tension clamp manufactured from spring wire according to the invention, the ratio Rm /Rp0.2 is regularly in the optimal range of 1: 1.2 for its service life and spring behaviour.
Nitrogen ("N") is provided in the spring steel of a spring wire according to the invention in contents of 0.0040 - 0.0120% by weight (40 — 120 ppm) in order to enable the formation of vanadium nitrides or vanadium carbonitrides.
However, excessively high N contents would promote the corner aging of the spring wire according to the invention, which would diametrically oppose the toughness of the spring wire according to the invention and the fatigue strength required by a tension clamp.
Negative effects of the presence of N in the spring steel of a spring wire according to the invention can be particularly reliably excluded in that the N content is limited to at most 0.0100% by weight (100 ppm). A spring wire consisting of a spring steel composed in a manner according to the invention achieves, in the hot-rolled state, a reduction of area at fracture Z of at least 55% determined in the tensile test according to DIN EN ISO 6892-1 and is therefore regularly higher than the reduction of area at fracture that can be determined in spring wires made of a conventionally alloyed 38Si7 steel.
At the same time, in the hot-rolled state, it has a fine granularity of its microstructure of atleast ASTM 10 determined according to ASTM E112. This fineness of the microstructure is largely retained by cold forming the spring wire into a tension clamp and the subsequent heat treatment of the tension clamp. Thus, tension clamps according to the invention, finished for installation in a rail fastening point, regularly have a fineness of their microstructure, which, determined according to ASTM E112, corresponds to at least ASTM 8. This corresponds to an improvement of the fine granularity by at least one of the granularity classes indicated in ASTM E112 compared to a tension clamp, which is bent from a spring wire made from the conventional 38Si7 steel. The method according to the invention for manufacturing a spring wire provided according to the invention comprises the following work steps: a) melting a steel, which consists of, (in % by weight), C: 0.35 - 0.42%, Si: 1.5 - 1.8%, Mn: 0.50 - 0.80%, Cr: 0.05 - 0.25%, Nb: 0.020 - 0.10%, V: 0.020 - 0.10%, N: 0.0040 -
0.0120%, Al: < 0.03% and as the remainder of iron and unavoidable impurities, the total content of impurities being limited to at most 0.2% and including impurities up to 0.025% P and up to 0.025% S; b) casting the steel into a primary product; c) hotrolling the primary product into a hot-rolled spring wire with an end diameter of 9 - 15 mm, wherein the hot rolling is carried out in at least two partial steps, wherein the spring wire is finished hot-rolled thermomechanically in the last partial step of the hot rolling at a temperature which is below the recrystallisation stop temperature of the steel of the spring wire and above the Ar3 temperature of the steel of the spring wire; d) cooling the thermomechanically finished hot-rolled spring wire at a cooling rate of 1 - 5°C/s to a winding temperature of 550 — 650°C; €) placing or winding the spring wire cooled to the winding temperature into a coil;
f) cooling the spring wire in the coil to room temperature.
According to the invention, the spring wire is thus subjected to a thermomechanical rolling step in the course of hot rolling, at which it is rolled at temperatures which are rolled below the recrystallisation stop temperature and above the Ar3 temperature of — the steel.
The temperature at which the spring wire has cooled down so far that recrystallisation of its austenitic microstructure up to that point no longer takes place is referred to as the “recrystallisation stop temperature”. The thermomechanical rolling carried out in the temperature range specified according to the invention in combination with the alloy selected according to the invention, in particular as a result of the simultaneous presence of Nb and V, results in the particularly fine-grained microstructure, which characterizes a spring wire according to the invention in the hot- rolled state.
At the same time, by cooling the hot-rolled spring wire at the cooling rates specified according to the invention and by maintaining the winding temperatures of 550 —
650°C prescribed according to the invention, itis ensured that a maximum hardness of the spring wire according to the invention is achieved as a result of precipitation hardening.
In principle, it would be conceivable to carry out the hot rolling partial step “thermomechanical rolling” in a separate work step, which is carried out after the actual hotrolling of the spring wire.
For this purpose, the provided spring wire, which is then hot-rolled, is first heated to the austenitisation temperature, then cooled to a temperature below the recrystallisation stop temperature, but above the Ar3 temperature of the spring steel and hot-rolled at this temperature with a sufficient degree of deformation.
The cooling and the laying or winding of the spring wire then
— takes place as indicated in the work steps d) and e) of the method according to the invention.
However, a technologically and economically optimised variant of the method according to the invention envisages all partial steps of the hot rolling (work step c))
being completed in a continuous process, i.e. a spring wire which is also thermomechanically finished hot-rolled when the spring wire leaves the respectively used hot rolling section . The invention is explained in greater detail below using exemplary embodiments.
In accordance with the invention, alloyed melts E1-E5 were melted, the compositions of which are indicated in Table 1. For comparison, a comparative melt V1 was melted, the contents of which were C, Si, Mn, P, Sand N in accordance with the specifications applicable to the known steel 38Si7, but which also had Cr in an effective content.
The composition of the comparative melt
Vlisalsoindicated in Table 1. Conventional bars have been cast from the melts E1 - E5,V1, which have also been pre- rolled and intermediately rolled into spring wires in several steps in a conventional manner before they have been finished hot-rolled in a last step of the hot rolling.
This last step of the hot rolling was performed as thermomechanical rolling.
For this purpose,
— the spring wire, before entering the last hot rolling step, was cooled to a temperature below the recrystallisation stop temperature of the steels E1 — E5 and V1 in the range of 850 - 950°C and above the Ar3 temperature of the steels E1 - E5 and V1 in the range of 750 - 800°C.
The recrystallisation stop temperature of the respective spring steel from which the
— respective spring wire E1 - E5, V1 is produced can be determined experimentally in a manner known per se or estimated using empirically determined formulas.
Similarly, the Ar3 and Ar1 temperatures of the respective spring steel from which the respective spring wire E1 - E5, V1 is produced can be determined experimentally in a manner known per se, for example by means of dilatometry in a thermomechanical simulator.
At the end of the hot rolling, the hot-rolled spring wires obtained were cooled ata cooling rate of 1 — 5°C/s to a winding temperature of 550 — 650°C at which they were wound into a coil. The spring wires in the coil were then cooled to room temperature. The grain fineness "ASTM F" of the microstructure was determined on the hot-rolled spring wires obtained in accordance with ASTM E112 and the reduction of area at fracture "Z F" in accordance with DIN EN ISO 6892-1. The values obtained "ASTM F" and "Z F" are indicated in Table 2 for the spring wires consisting of steels E1 - E5 and
V1. From the hot-rolled spring wires consisting of the spring steels E1 - E5, V1, rods have been cut to length which have been bent in several steps, cold, i.e. at room temperature, into a conventionally formed, w-shaped tension clamp after pickling and straightening in a conventional manner. After this cold forming, the tension clamps obtained were subjected to a heat treatment in which they were heated to an austenitisation temperature of 850 - 950°C, so that their microstructure was completely austenitic. The austenitised tension clamps were then quenched in water so that their microstructure was more than 95% by area martensitic. After quenching, the tension clamps have undergone a tempering process during which they have been heated to a tempering temperature of 400 — 450°C over a period of 60 - 120 min and held there. The tension clamps tempered in this way were then cooled to room temperature in air. The tensile strength Rm and the yield strength Rp0.2 have been determined on the tension clamps obtained in this way in accordance with DIN EN ISO 6892-1. In addition, notched impact energy KV-20 has been determined in accordance with DIN ENISO 148-1 as a characteristic value for toughness. The measured values obtained are listed in Table 2. It was found that not only the tensile strength Rm and the yield strength Rp0.2 of the tension clamps produced in the manner according to the invention from spring steel E1 composed according to the invention could be significantly increased with unchanged notched impact energy KV-20 compared with the tension clamps manufactured from the comparative steel V1, but also that the ratio Rm /Rp0.2 has remained practically the same.
Atthe same time, the tension clamps produced from the spring steels E1 - E5 according to the invention had a significantly improved fine granularity “ASTM” of the microstructure determined according to ASTM E112 than the tension clamps consisting of the comparative steel V1.
The tension clamps consisting of the steels E1 - E5 according to the invention and the comparative steel V1 were then installed under identical conditions in a fastening point and the hold-down forces exerted by them were determined in new condition “TLn” and after 3 million load changes "TL3m". The results of this measurement are also indicated in Table 2. It can be seen that the tension clamps consisting of the spring steels E1 — E5 according to the invention not only deliver a higher hold-down force TLn in new condition, but that this hold-down force only decreases slightly even after 3 million load changes, whereas it decreases by a significantly larger amount in the tension clamps consisting of the comparative steel V1.